This invention relates to a positive-working resist composition, a pigmented formulation of a positive-working resist composition and to a method for using this pigmented formulation for forming a light-absorbing matrix in color cathode ray tube screens.
Color cathode ray tubes (CRT) such as employed in color television applications generally have a patterned screen comprising repetitive groups of phosphor material which may be in the form of bars, stripes or dots. Thus, the well known shadow mask tube construction may be employed wherein the screen pattern is in the form of dots formed of selected cathodo luminescent phosphors which phosphoresce by predetermined electron excitation, to produce additive primary hues in order to produce the desired color emanations.
This electron excitation of the phosphor dots is achieved by focusing the electron beams from three electron guns through a shadow mask positioned in a spaced manner from the screen. The shadow mask is provided with apertures each of which is perceptively related to a specific grouping of similar shaped dots in the screen structure to enable selected electron beams traversing the apertures to impinge on the proper phosphor dots on the screen. These dots are generally separated by relatively small interstitial spacings in order to enhance color purity by reducing the possibility of electronic excitation of adjacent dots.
In order to enhance the brightness, color purity and contrast of the resulting color screen image the interstitial spacing between the phosphor dots, stripes or bars is generally occupied by an opaque light-absorbing material or a matrix which generally is a black pigmented material.
Hedler et al, U.S. Pat. No. 3,658,530 describes a frequently employed method for providing such a light-absorbing matrix on a screen of a color CRT.
According to the method described in this patent the interior surface of the screen is coated with a thin uniform layer of a polyvinyl alcohol solution made photosensitively active with an hexavalent chromate material. The thus coated screen is then exposed to light through the apertures of a pattern mask thereby polymerizing the exposed portions of the photosensitive polyvinyl alcohol layer. The exposed coating is then soaked with water to remove the unexposed and resultant unpolymerized areas of the polyvinyl alcohol layer to thereby provide a plurality of repetitive elements of polymerized polyvinyl alcohol dots separated by an interstitial pattern or web of substantially clear glass.
This panel is then coated with an opaque colloidal suspension of graphite and then dried to form an opaque graphite film, the opaque graphite film being present both on the bare glass and on the polymerized polyvinyl alcohol dots.
The panel is then treated with a hydrazo-reducing agent capable of degrading the polymerized polyvinyl alcohol dots. By this treatment the light polymerized polyvinyl alcohol pattern dots are degraded and the graphite film positioned on these dots is loosened from the screen.
The screen is then rinsed to remove the loosened degraded polyvinyl alcohol dots and the associated graphite and leaving thereby a multitude of clear glass windows surrounded by an opaque interstitial web or matrix of graphite.
The resulting window patterned screen is then dried and by means of conventional screening techniques the phosphors are applied to the respective window areas and a cathode luminescent screen is formed.
This method has the drawbacks in that the large number of steps in the process which requires manual handling provides opportunities for potential handling errors and inclusion of dirt in the screen.
Further the use of the hexavalent chromium provides a potential pollution problem as the amounts of this material and the hydrazo-reducing agents materials permitted in the effluent are extemely small.
Similar methods of forming the light absorbing opaque matrix are disclosed in Speigel, U.S. Pat. No. 3,822,454, particularly column 9, lines 20-60; and Bergamo et al U.S. Pat. No. 4,245,019, particularly column 4, line 37 - column 5, line 60 where a hydrogen peroxide solution is employed as the degrading agent.
While the use of hydrogen peroxide is advantageous over the hydrazide degrading solutions of the Hedler et al patent as being less hazardous, the use of hydrogen peroxide solutions are still undesirable as the hydrogen peroxide solutions are also hazardous and they must be closely controlled both for safety's sake and for proper image development. In addition, the presence of undecomposed hydrogen peroxide in the manufacturing plant effluent will reoxidize any trivalent chromium (produced by a previous light exosure process) to hexavalent chromium compounds.